Add an account block showing who is signed in

[us_account], or the Account block: the signed-in visitor's name, their
email, a Sign out link, and — only when the account books for someone
besides itself — the students it books for. A parent's first question on
seeing "signed in as Grace" is whether this is the account their children's
lessons are on.

Two decisions worth naming.

Signed out with no login page chosen, the block renders nothing. Its whole
subject is the person signed in, which a stranger is not, and a bare "you
are not signed in" in a site header is noise with no way to act on it. With
a login page chosen it offers a Sign in link instead. The editor preview is
populated regardless, so the block is never an invisible box to the person
placing it.

Signing out returns to the chosen login page, or to the current page when
there is none. A block meant for a header should not also navigate someone
somewhere when they use it; the login page wins when configured, because the
page they were on may well be members-only.

The name comes from UserName::format(), so the block never exposes a
username the way display_name can.

Also brings docs/features/editor-blocks.md back in step: it still described
"four shortcodes" and had never listed the family block.

Closes #142

Co-Authored-By: Claude Opus 5 <[email protected]>

CHANGELOG.md

@@ -13,6 +13,9 @@ each change under the current top section as you work.
 
 ## [1.3.1]
 
+### Added
+- An **Account** block (`[us_account]`) showing who is signed in — their name, their email, the students they book for if that is anyone besides themselves — and a **Sign out** link. Signing out returns to the login page chosen in the block, or to the page the visitor was already on when none is set, so putting it in a site header does not also move people somewhere. To a signed-out visitor it shows a **Sign in** link when a login page is chosen, and nothing at all when one is not: a panel about who is signed in has nothing to tell a stranger, and a notice they cannot act on is just clutter in a header.
+
 ### Changed
 - A student's **name and birth year are now required**, marked in the form the same way a required registration question is and enforced on the server whichever way they were submitted. On signup the requirement applies only once the parent/guardian box is ticked, so registering for yourself is unaffected. A student block you have started filling in is now reported back to you rather than silently dropped when the name is missing — only a completely untouched spare block is still ignored.
 - Signup and the profile page now ask for a **birth year** rather than a full date of birth — a four-digit year between 1900 and the current year, with anything else discarded rather than stored. Students added before this change keep showing a birth year, derived from the date already on file; that old full date is then dropped the first time the record is saved, so the studio ends up holding only what it now asks for. No bulk purge runs, so a site wanting the remaining old dates gone should clear the `us_date_of_birth` user meta directly.